He has done it in part by taking a long view of his role at the court, sticking to his convictions, and waiting - often for decades - until a majority of the court recognizes what he sees as the wisdom of his position.

For example, in the decision extending federal court jurisdiction to detainees at Guantánamo Bay, Justice Stevens firmly established as law a legal argument he first encountered in 1948 as a Supreme Court law clerk. His boss, Justice Wiley Rutledge, embraced the argument in a dissent.

But last week - 56 years later - the same legal principles expressed in that Rutledge dissent became the clear law of the land in a landmark decision Stevens penned.

His hand is also apparent in last term's landmark gay rights decision. Some of the same language used by Justice Anthony Kennedy in his majority opinion in Lawrence v. Texas stems from a 1976 decision written by a little-known Chicago-based appeals court judge. His name: John Paul Stevens.

The Stevens-Kennedy collaboration was no accident. Stevens used his power as the court's senior liberal justice to assign the gay rights opinion to Kennedy, opening the way for a blockbuster ruling reflecting a constitutional conclusion Stevens had reached 27 years earlier.

"I think it is the hidden story of the last several years," says David Barron, a former Stevens law clerk who now teaches at Harvard Law School. "In many ways it is becoming the Stevens Court."

As the second most senior justice after Chief Justice William Rehnquist, Stevens' influence comes more by default than design. But the Chicago native appears to have made the most of his opportunities, voting with the majority in the recently concluded 2003-2004 term in seven of the term's top ten cases.

In addition to authoring the Guantánamo Bay decision, he wrote the opinion dismissing the Pledge of Allegiance lawsuit of California atheist Michael Newdow, and teamed up with Justice Sandra Day O'Connor to pen the decision upholding key provisions of the McCain-Feingold campaign finance law.

Countering the conservatives

Stevens' influence was also on display in majority opinions that turned back efforts by the court's conservative wing in the areas of states' rights, police interrogation tactics, and government funding of religion.

"Justice Stevens emerged as a unifying and leading force on the court in part because in an array of important cases the conservative majority did not hold and that left him in control," says Thomas Goldstein, a Supreme Court advocate and analyst in Washington, D.C.

The conservatives weren't completely thwarted, largely prevailing in two of the terrorism cases and issuing a string of opinions favoring law enforcement. But the big wins of the term belong to liberals and moderates, suggesting a Stevens role behind the scenes.

Under court tradition, shortly after oral argument in each case the justices vote on how the case should be resolved. If Chief Justice Rehnquist is in the majority, he decides who will write the opinion. If not, the assignment is made by the most senior justice in the majority. Frequently that is Justice Stevens.

It isn't enough to merely assign the author. The key to exerting influence within the high court is the ability to hold the majority during the opinion writing process. Sometimes this is done by assigning the case to the justice who is most likely to jump to the other side should the opinion be written too narrowly or too broadly. Sometimes the assigning justice assigns the case to himself and then assumes the role of judicial diplomat, taking care to adequately address the concerns of each member of the majority.

It is in this role as a kind of shadow chief justice that Stevens has undercut the agenda of the court's more conservative justices. He has done it by appealing to the court's two moderate- conservative swing voters, Justices O'Connor and Kennedy.

But another key to his influence is his ability to appeal for votes more broadly across the court's usual ideological divide. Two criminal law decisions this year illustrate the point.

In Blakely v. Washington, the court extended the Sixth Amendment right to a jury trial in a way that raises significant concerns about the continued viability of federal sentencing guidelines. The five-justice majority included conservatives Clarence Thomas and Antonin Scalia. Justice Scalia's opinion echoes a position raised in a Stevens dissent written 18 years earlier.

In Crawford v. Washington, the court put new teeth into the Sixth Amendment right to confront witnesses by barring prosecutors from introducing certain kinds of recorded statements as evidence at trial. Again the decision was written by Justice Scalia. It essentially laid out a position embraced by Stevens 25 years ago. The vote in the case was 9-0.

Marci Hamilton, a law professor at Cardozo Law School in New York City, told a recent gathering of the American Constitution Society that 2003-2004 had been a "remarkable" term for Stevens.

"He has brought together in a wide variety of decisions majorities that you never thought would coalesce in those cases," she says. "Stevens is starting to look like [the late Justice William] Brennan in his later years where he's more interested in finding a majority and consensus than he is in a particular extreme viewpoint."

Is it still a Rehnquist Court?

But other court watchers are reluctant to read too much into the recent term. "It is too early to tell if this is a major shift," Mr. Goldstein says. "It is unlikely that we are seeing anything that will cause us in 10 years to refer to the Stevens Court rather than the Rehnquist Court."

Some analysts see the recent frustration of the conservative wing's agenda coming more as a result of the actions of Justices O'Connor or Kennedy, rather than Stevens.

"We have reached the endpoint, for now, of the court's conservative jurisprudence on all the hot button issues - state's rights, property rights, abortion, gay rights," Goldstein says. "Over the last few terms the social conservative doctrine has been turned back every time. It has gone as far as Sandra Day O'Connor will let it."

Others say Stevens has long been overlooked as an influential force on the court. "He is a very modest person and he doesn't convey to you that he is a politician looking for votes," says Professor Barron.

"But when you look at the record of assignments he's made and the way he has crafted opinions," Barron continues, "it is really quite striking that he has been able to take the Rehnquist Court in a direction that I imagine Chief Justice Rehnquist did not want to go."